Pregnenolone was identified as the major product when progesterone was incubated with cultures of Tetrahymena pyriformis. This transformation is the first demonstration of reversal of the 5-ene-3beta-hydroxy yields (reversibly) 5-ene-3-ketone yields (reversibly) 4-ene-3-ketone pathway in an intact organism. Our objective is to characterize the nature of this dehydrogenase-isomerase system of T. pyriformis by purification of the dehydrogenase and isolation of the pathway intermediates. A growth inhibition study will also be done to investigate the hypothesis that these transformations are related to the toxicity of steroids to this protozoan. BIBLIOGRAPHIC REFERENCE: Lamontagne, N.S., Will, A.R., Johnson, D.F., and Holmlund, C.E.: The Conversion of Progesterone to Pregnenolone by Tetrahymena pyriformis. J. of Steroid Biochem. 8: 329, 1977.